A membrane bioreactor used at the occasion of treatment of sewage and industrial wastewater is a treatment method in which biological treatment is performed in a biological reaction tank and activated sludge is subjected to a solid-liquid separation by the use of filtration membranes submerged in the reaction tank to obtain clear treated water.
In the membrane bioreactor, a membrane surface is always cleaned by air supply from beneath a separation membrane unit. However, when operation is continued for a long period of time, sludge-derived matter which cannot be removed by only the air supply sometimes remains on the membrane surface. When this matter once starts to adhere thereto, the effective surface area of the membrane is lowered, sludge deposition grows based thereon, and the thickness or the area is gradually increased to soon form a sludge layer called a cake layer. In this state, the sludge layer cannot be removed by only the usual air supply, and therefore, first, a method of performing chemical liquid cleaning with sodium hypochlorite is considered.
As the chemical liquid cleaning method of membranes, there are an out-of-tank cleaning method in which a submerged type membrane separation unit in its entirety or a membrane element is taken out from a tank and cleaned, and an in-tank cleaning method in which a chemical liquid is injected into a membrane permeate channel while submerging the separation membrane unit in a tank. However, particularly in a flat sheet membrane unit, the latter in-tank cleaning method is mainly used, because of problems of workability and space. Patent Document 1 proposes a method in which a chemical liquid for decomposing matter adhered to membranes is injected into a permeate channel of a membrane separation device and into a membrane element, and a state that the chemical liquid in the permeate channel is in contact with the filtration membranes is maintained for about 1 hour.
Furthermore, the adhered matter which causes clogging of the membranes in the membrane bioreactor includes not only organic matter not decomposed by the activated sludge, but also inorganic matter contained at a high concentration in water to be treated, and so on. In such a case, there is a method in which chemical liquids different in effect are used in combination in response to the adhered matter. For example, Patent Document 2 and Patent Document 3 propose in-tank cleaning methods in which sodium hypochlorite for decomposing the organic matter and hydrochloric acid, citric acid, oxalic acid or the like for removing the inorganic matter are used and sequentially injected in two steps.
All of these methods have effects of effectively removing the matter which causes clogging of the membranes, such as the organic matter and the inorganic matter, and recovering the permeation flux of the membranes.
However, after the sludge is once adhered to the membrane surfaces firmly or in large amounts, the membranes are sometimes only partially recovered even when the chemical liquid cleaning is performed. When the chemical liquid concentration is increased or the chemical liquid submersion time is prolonged, in order to enhance the cleaning effect, a part of the chemical liquid injected flows out into the tank, which causes a problem of exerting adverse influences such as lowering of physiological activity of microorganisms in the activated sludge and deterioration of filterability by decomposition of the sludge. On the other hand, under mild chemical cleaning conditions, it is necessary to repeat cleaning many times, and finally, there is nothing but to take the submerged type membrane separation unit out from the tank and to clean flat sheet membrane elements one by one. This causes a problem of large-scaled and complicated work.